The invention relates to an input buffer for a switched emitter follower-like track-and-hold amplifier comprising an input stage with an input transistor, a first diode, a cathode side of which first diode is connected to an emitter of the input transistor, a first current source between on the one hand the junction between the cathode of the first diode and the emitter of the input transistor and on the other hand a first supply voltage line, an anode of the first diode being connected to a track-and-hold controlled emitter follower.
Such an input buffer is known from U.S. Pat. No. 5,583,459. Input buffers described therein have a current source connected between the anode of the first diode and a second supply voltage. The presence of a diode is crucial to disconnect the input signal during the hold-mode.
It is known to embody the current source between the anode of the first diode and the second voltage supply with MOSFETs. A problem with such a current source embodied with MOSFETs is the nonlinear junction impedance thereof. When high input frequencies are used as input signals to this input buffer it is especially the nonlinear junction capacitance that plays a prominent role, at lower frequencies the nonlinear junction resistance also will play a role. The nonlinear junction impedance leads to distortion of the signal at the anode of the first diode. That distortion cannot be prevented by increasing the current through the current source since more current requires a proportionally larger current source with inherently larger nonlinear junction impedance. Furthermore a strict trade-off exists between available voltage headroom, noise and nonlinear output impedance of the current source.